1. Field of the Invention
The invention relates to a method and apparatus for forming metal castings having fine grained structures.
2. Description of the Prior Art
In casting metal articles it is sometimes important that a cast billet be formed which may subsequently be worked so as to impart desired strength and other structural properties. Thus, for example, it may be necessary to subject cast billets to a hot working process by forging them several times to strengthen them. In those cases where the grain size of the billet is large, the hot working process may itself involve several steps. This, of course, is both costly in terms of time and energy. Also, by virtue of the large grain sizes often present in castings and as a result of the lengthy but necessary forging and rolling processes, the casting has a tendency to crack thus making it commercially undesirable.
Thus, if a satisfactory method of commercially producing fine grain castings were available it would be possible to simplify the hot working operations as well as reduce costs while obtaining a product of improved quality.
Known methods of making fine grain castings include casting atomized molten metal as well as casting the molten metal after it has partially solidified.
The atomization technique essentially comprises using an inert gas to atomize a molten metal and then catching the atomized metal in a container just before it has become solidified.
Atomization techniques have proven unsatisfactory since some of the inert gas used to atomize the metal becomes trapped within the final solidified metal billet thus lowering its quality.
When casting molten metals after they have partially solidified it is necessary that the temperature throughout the process be carefully controlled and kept constant while the metal, in a "mushy state", is being poured. The necessity for careful process control inherently complicates the operation.
Attempts at what is known as "drip casting" have involved the use of consumable electrodes which are heated to supply molten alloy. The alloy is then passed into a tundish or holding induction pot from which it is poured into a water cooled mold. However, use of a tundish requires that the tundish be preheated so as to prevent premature freezing of the molten metal. A process and apparatus such as this is illustrated by FIG. 3 of U.S. Pat. Nos. 3,847,205 and 3,920,062, the disclosures of which are hereby incorporated by reference. The tundish thus complicates the process and apparatus by the addition of a costly extra step which must be carefully controlled and monitored.